feat: switch notification playback to Web Audio API to avoid macOS media session

[Dimillian]

Motivation

• Playing notification sounds with new Audio(...).play() registered the app with macOS Now Playing / Media Control Center and could cause device focus switching; the change removes that behavior by using the Web Audio API.

Description

• Replace use of HTMLAudioElement in src/utils/notificationSounds.ts with a Web Audio API implementation that fetches, decodes, and plays an AudioBuffer via a BufferSource and GainNode at low gain.
• Add lazy AudioContext resolution/reuse with support for AudioContext and legacy webkitAudioContext, and call resume() when the context is suspended before playback.
• Preserve and consolidate debug logging for initialization/load/play failures and update error labels to audio/<label> load/play error.
• Add a focused unit test suite src/utils/notificationSounds.test.ts (jsdom environment) covering successful playback, fetch failure debug logging, and suspended-context resume behavior, and replace global usage with globalThis where needed for typecheck compatibility.

Testing

• Ran the focused tests with npm run test -- src/utils/notificationSounds.test.ts, which passed (3 tests passed).
• Ran npm run typecheck and the full test suite with npm run test, and the typecheck and full suite completed successfully (typecheck passed and all tests passed).
• Addressed an initial test failure caused by a missing jsdom environment by adding // @vitest-environment jsdom to the new test file, after which the tests passed.

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Codex Task

[Dimillian]

@codex review

[chatgpt-codex-connector]

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